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Hubbell Power Systems Engineers Recently Gave a Presentation at GeoCongress 2020 on Repairs and Replacements to an Xcel Energy Transmission Line
The Xcel Energy W3408 69KV overhead transmission line was originally built in 1934 and extends from the Nelson Substation in Wisconsin to the Wabash Substation in Minnesota. This transmission line crosses the Mississippi River and its backwaters through the Federal Upper Mississippi River National Wildlife Refuge.
Due to age and condition Xcel energy identified 2.2 miles of direct embedded wood H-frame structures in Wisconsin for rebuild in 2018 through 2019. There were numerous unique design challenges due to the location and existing conditions. These design challenges included 2 to 3 feet of standing water, winter ice, environmental concerns, limited outage times, limited access, and poor soil conditions.
The chosen principle foundation consisted of groups of four RS3500.300 CHANCE ® Helical piles connected by a steel grillage at each structure leg. Theoretical installation depths ranged from 52 to 62 feet and were designed for up to 6.5 feet of scour potential. All material could be transported to the structure locations by airboat and installed using marsh equipment. This system minimized outage times and allowed the project to meet the tight scheduling constraints of the project. Structures were set in the fall of 2019 using heavy-lift helicopters to reduce environmental impacts, construction duration, and cost.
The entire paper, entitled “Use of Alternate Foundation to Overcome Design and Construction Challenges in Mississippi Backwater” was jointly written by Simon Murley, PE (Power Engineers), Christopher Strom, PE (Xcel Energy), and Jason Herron, PE (HPS). It is available for purchase from ASCE.